Selective radio receiving system



Dec. 28, 1937. I E, THQMPSQN 2,103,878

SELECTIVE RADIO RECEIVING SYSTEM Filed April 30, 1936 2 Sheets-Sheet. l

INPUT Bnventor jclarzdzfsom (Ittorneg Dec. 28, 1937.

L. E. THOMPSON SELECTIVE RADIO RECEIVING SYSTEM Filed April 50, 1936/0KC. 40KC.

. AF. 05 Awe Zhwentof Patented Decs28', 1937 PATENT OFFICE SELECTIVERADIO RECEIVING SYSTEM Leland E. Thompson, Merchantville, N. J.,assignor to Radio Corporation of America, a corporation of DelawareApplication April so, 1936, Serial No. 77,253

16 Claims. (Cl. 2502Q) The present invention relates to selective radioreceiving systems and has for its object to pro- ),vide a selectiveradio receiving system wherein the reduction of static and interferencefrom undesired signals with a predetermined desired signal may beprevented or materiallyreduced for telegraph, continuous wave andvtelephone signal reception. This invention is an improvement on theradio telegraph and telephone receiving system shown and described in myU. S. Patent 1,929,036. i d

Inthe system disclosed in the patent referred to, a static limitercircuit comprising a balanced detector is arranged to make use of theprinci- 5 ple of aheterodyne straight line detector wherein the outputbeat frequency between two input voltages of differing frequency has anamplitude dependent only upon the amplitude of the smaller of the inputvoltages and is independent of the larger voltage whereby the circuitoperates as a limiter when the smaller voltage is supplied by a localoscillator and the larger voltage is provided by disturbing effects suchas static or an interfering radio signal wave.

In the system referred to broadly responsive tuned circuits are providedin advance ofthe static limiter or balanced detector to. obtain betterstatic or noise reduction since the length of the oscillation trains insuch circuits are materially reduced. Such a receiver, however. is notselective to radio signal waves and, accordingly, it is a further objectof the present invention to provide a radio receiving system whichimparts further selectivity to thesystern whereby signal waves inadjacent frequency channels maybe selected without interference.

It is also a further object of the present inmention to provide animproved interference limiting system which provides for the reductionof interference to a greater degree than that which is ordinarilyprovided by a static limiter circuit broadly tuned over a wide band, anda higher degree of selectivity than such systems provide.

The invention will, however, be betterunder stood from the followingdescription when con-. sidered in connection with the accompanyingdrawings and its scope will be pointed out in the appended claims.

In the drawings, Figure 1 is a schematic circuit diagram of a radiosignal receiving system embodying I the invention;

Fig. 2 is a curve indicating the frequency characteristic of thereceiving circuit and signal voltages provided in the system of Fig. 1in the operation thereof;

sence of oscillations.

Fig. .3 is a series of curves showing the modulation envelope resultingfrom the reception of: an interfering wave in the system of Fig. 1; IFig. 4 is :a schematic circuit diagram of a radio receiving systemembodying the invention being a modification of the circuit of Fig. 1;

Fig. 5 is also a schematic circuit diagram of! a radio receiving systemembodying the invention and being a modification of the circuit. of 3Fig.1 forthe reception of telephone or modulated carrier wave signals,and. v Fig. 6 is a curve showing the frequency re-' 'sponsecharacteristic and operating voltages of the circuit of Fig. 5.

Referring to Fig. 1, a radio receiving system is shown wherein 5 is anintermediate frequency amplifier comprising a plurality of tunedcirtrodes l2 and I3 associated therewith.

The rectifier circuit includes the tuned circuit 9, the high potentialside M of which is connected with the cathode II and the low potentialside of which is connected through a lead I5 to. ground [6, and to eachof the anodes I2 and I3 through a separate output resistor l1 and I8.The output resistor 18 is connected directly with the anode I2 and theground l6 and the lead l5 through a circuit'lead l9 and the resistor I1is" likewise connected with the anode I 3 and'the ground l6 and the lead15 through a connection lead 20 and a coupling coil 2|.

The coupling coil 2| provides a signal input connection for oscillationsderived from an OS- "sistors energy having the same wave form and thesame phase relation between current and voltage, whereby zero output maybe obtained by tapping at equiepotential points in the ab- An' audiofrequency output circuit is provided in connection'with two variable tapconnections 28 and 29 forming the terminals of the primary winding 30 ofan audio frequency coupling transformer 3|. The resistors 11 and itprovide two balanced output circuits on which equi-potential points maybe found whereby in response to received. carrier waves the signaloutput applied to the output transformer winding 30 may be substantiallyzero. However, as described in the patent referred to the contacts 28and 29 may be adjusted with respect to each other whereby the beatfrequency between the oscillator and an incoming signal may be appliedto the output transformer.

For example, a continuous wave signal will produce an audio frequencybeat voltage with the oscillator voltage introduced at'2l which willappear across the resistor I! but not across the resistor i8. It willthen be present across. the audio frequency transformer primary 30. andthis voltage will at all times represent the difference between thepotential at 28 and the potential at 29-. may have any voltagepreferably not greater than the signal voltage or carrier Voltage and inany'case not greater than twice the peak voltage of the received carrierwave for 100% modulation if the carrier wave is modulated.

' If the oscillator'voltage is equal to or less than the incoming signalvoltage the output beat frequency between the two input voltages willhave an amplitude dependent only on the amplitude V of the smaller ofthe input voltages which is the ing circuit preceding the detector i0.

oscillator voltage, and'this will be independent of the, larger orsignal voltage.

The signal receiving circuits preceding the balanced detector it arepreferably broadly tuned and may be, in the present example, as broad as50 kc., in which case an incoming interference Wave of greater thansignal intensity may operate to cut off the desired signal as will beseen by'referring to Fig. 3 along with Fig. 1.

In Fig. 2 the vertical line A represents the frequency and amplitude ofa signal impressed on the detector Iii of Fig. l and the vertical line Crepresents thefrequency and amplitude of the 7 local oscillator voltageimpressed upon one-half of the balanced detector circuit. 7 The vertical7 line B represents an interference voltage such as a continuous wavesignal voltage impressed with the incoming signal on the detector 10;The curve E represents the over-all selectivity of the intermediatefrequency amplifier or receiv- It will be noted that it is relativelybroad and maybe of a value as indicated having a band width of 50 kc. ormore.

If now the beat frequency between A and C is 500 cycles and the beatfrequency between A'and B is 6000' cycles the 500 cycle frequency is theaudio signal frequency which is desired. The voltages across theresistor I H which is one branch of the balanced detector circuit areshown in, Fig. 3'to which attention is now directed along withFigsl and2.. I

Referring to Fig. 3 the straight line detector characteristicof thedetector I0 is indicated by the line E'and the curve AB represents theundesired 6000 cycle voltage. This volt'age is'modulated at the desiredaudio frequency signal fre- The oscillator voltage introduced at 2| Inthe present example, the audio frequency.

coupling transformer 3| provides the input coupling means for the audiofrequency amplifier stage following the balanced detector. Thiscomprises a single amplifier tube 31 of the electric discharge typehaving an output circuit 38 coupled through an audio frequency couplingtransformer 39 with the circuit 35.

The detector circuit 35 may include a single or a push-pull or (fullwave) detector and in the present example it is of the latter type andincludes an electric discharge amplifier device Mi having a cathodeandtwo diode anodes 42 associated therewith and'connected with thetransformer secondary winding 53 at the terminals thereof. The centertap of the balanced input circuit, indicated at 44, is connected througha diode rectifier output resistor 55 vwith the cathode ii and ground 46.'In this manner a full wave rectifier circuit is provided in connectionwith the output resistor 45:

The output device 36 is coupled to the balanced detector circuit throughan audio frequency ampli'fier 47, the initiai'stage of which is alsoprovided in the device 43 through the medium of a control grid 33 and anoutput anode 3:1, the latter being connected with the amplifier 41, andthe control grid being connected through a filter resistor 58 with thenegative terminal 45 of the diode output resistor 45 whereby the gridreceives biasing and signal potentials therefrom. The output resistor isprovided with a suitable bypass capacitor 50.

It is desirable that the transformers 3i and 39 have relatively broadfrequency characteristic suficientto carry through to the auxiliarydetector circuit, 35 both the modulation and the carrier waverepresented by the signal AB and the modulation AC of Fig. 3 for thereason that unless this is provided'the carrier wave will be cut off anddistortion willrresult. M both of these frequencies are well within theaudio frequency range, suitable transformers may be obtained having aflat characteristic at such frequencies. This is true even though thefrequency AB be as great as the frequency band passed by the tuned inputcircuits although in general it is preferable to maintain this frequencywithin the audio frequency range as described. 7

From the nature of the balanced detector circuit it will be seen thatifthe interference voltage B of Fig. 2 varies, the effect upon theoutput will not change so long as the voltages A and C are lower andwill have only th efiect of shifting the envelopes AB and AC to theright with increased interference voltage in connection with the signalinput. envelope of Fig. 3 and in a vertical direction from the zero axisin the case of the signal output envelope of Fig. '3. It will be seenthat the desired signal AC will not be recorded unless detected later tocut off the lower half of the envelope of the output wave shown in Fig.3. By the use of an additional detector or rectifier connected with abalanced detector limiter the modulation or desired signal may bederived in the presence of an interference wave in the same manner thatit is received in the absence of such wave.

Briefly, the receiving system shown and described provides for receivingsignals and limiting disturbing effects therein in an amplifier having arelatively broad frequency characterstic preced ing a detector. Thedetector includes two output circuits opposed to each other wherein theenergy in each of the opposed circuits may be of the same wave form andhave the same phase relation between current and voltage. This is mosteasily provided by the'resistor output circuits'provided with suitableby-pass capacitors one of which is adjustable. The received energy istransformed in one of the output circuits by combining with locallyproduced oscillations which are maintained appreciably smaller than theI amplitude of the received energy. The resultant signal frequency isderived by further amplification, and rectification of the output fromthe balanced. detector circuit whereby in the presence of a disturbancesuch as a carrier wave of undesired frequency the modulation of thebeatfrequency between the undesired and desired signal waves may be derivedas the desired signal and may be suitably amplified and reproduced.

Referring now to Fig. 4, the circuit of Fig. 1 is shown in part, and forthe same circuit elements the same reference numerals are used as inFig. 1. In this system, the auxiliary detector or rectifier, followingthe signal detector I0, is placed between the audio frequency outputtransformer 3| and the detector l0, and in this circuit comprises a pairof diode rectifiers 60 and BI, connected through coupling capacitors 62and 63, respectively, with the balanced output resistors l1 and H3. Therectifiers 60 and BI are also provided with input coupling impedances'provided by resistors B4 and 65, and with output resistors 66 and 61,preferably of equal resistance value, to which output resistors leads 68and 69 for the transformer 3| are connected at suitable tap points 10and I I, one of whichis preferably variable.

This system has the advantage that the transformer 3|, following thebalanced detector 60,6l, may have a sharp audio frequency band passcharacteristic, thereby lending selectivity to the system, and otherwiseoperating in a similar manner to the system shown in Fig. 1. "Byconnectfrequency band-pass input to the balanced deing the impedancecoup-ling connections, through the coupling capacitors 62 and 63, withequal impedances l1 and I8, only the modulated AC is transmitted to thebalanced detector 60, 6 I, and

in the case of an interference signal producing the wave AB, theoperation is substantiallyas shown in Figs. 2 and 3, and as described inconnection with Fig.1. 7 r

A desirable feature of Fig. 4 is that a greater tector may be used. (say50 kc.) and it will not be necessary to have an audio transformer thatwill pass this high frequency. (That is, if AB was 50 kc. it would beimpractical to design a transformer that would pass this frequencyefiiciently to the detector 35 in Fig. l.)

Referring now to Fig.5, the receiving system of Fig. 4 is shown asadapted for receiving a modulated carrier wave signal, and like partsthroughout bearthe same reference numerals as in Fig. 4. I

The desired modulated signal is receivedin the balanced detector HI, andthe resultant signal, after .combining with oscillations from theoscillator 22, are rectified in the balanced detector circuit 6|], GIand applied to a'tuned output circuit for the last named detector,provided by two tuned coupling circuits 8!] and 8! in connection with ahigh frequency coupling transformer 82. suitable amplifier 83,- and thedesired modulation is derived in an audio frequency detector 84. The

The-output signals are then amplified in a i detector signals areamplified in a suitable audio frequency amplifier 85, and applied to anoutput such that it lies outside of the overall response.

characteristic or selectivity E of the receiving system, preceding thedetector l0,.while A'and B representthe incoming modulated signal andthe interference voltages, respectively. AC. is shown at 40 kilocyclesmerely by way of example, and the amplitude: of the incoming signal A ispreferably'maintained somewhat below one-half that of the localoscillator, in order that when the signal A is modulated the peaks donot rise above the value of C; otherwise distortion may result. Thefrequency of the oscillatorC should be sufficiently far removed from theselectivity curve E that direct' audio frequency beat notes between thesignal and the oscillator are not obtained The input signal to theauxiliary detector. or rectifier 60, 6| will contain two signalfrequencies-that is, AB,-equal to 10 kilocycles, and BC, equalto 50kilocycles. The output circuit of the rectifier 60, 6|, as applied tothe transformer 82, will include, in addition, a third signal frequency,which is the difference between BC and AB, or 40 kilocycles, togetherwith a fourth signal frequency which is the sum of BC and AB, or

. 60 kilocycles.

"of the oscillator voltage C, regardless of the amplitude of theinterference B. This is for the reason that variations in the amplitudeof B servemerely to move the modulation envelope of AB, as provided byAC, to a greater distance from the zero axis as explained in connectionwith Fig. 3.

However, since the signal A is modulated, this modulation will appear onthe 40-kilocycle signal and the band-pass characteristic of the tunedcoupling circuits 8!] and 8| should be sufficiently broad to carry thismodulation. Therefore, a 40-kilocycle modulated signal is obtainedthrough the transformer 82, whether interference B is present or not.

The 40-kilocycle modulationsignal is then amplified in the amplifier 83,if desired, and is finally detected in-the detector 84 to derive thedesired modulation or audio frequency signal.

From the foregoing description, it will be seen that a selectivereceiving circuit in accordance work.

with the invention comprises relatively broadly tuned receiving andamplifying circuits, preceding a balanced diode rectifier detectorhaving a balanced output circuit in one branch of which' is connected asource of oscillations to provide a beat frequency. The rectifier isconnected with the output circuit of the balanced detector, and arrangedto rectify or detect the beat frequency alone or after the modulationenvelope onan interfering signal wave, whereby the presence ofinterference is prevented from interrupting the reception of, a' signalwave.

It will be seen that the rectifier following the balanced detector maybe placed in advance of or following the audio frequency couplingtransformer, and that for modulated carrier wave signals thedetectorfollowing the balanced rec'- tifier or first detector isconnected withan audio frequency detector through a selective tunedsignal circuit responsive to the beat frequency between the oscillatorand the desired received carrier wave. i

The diode type .of detector or rectifier is preferred, as shown herein,for the reason that it insures straight line rectification in both thelimiter and the demodulator or output rectifier stages, which type ofrectification is substane tially necessary or highly desirable for mosteffective operation of the limiter system. 7

From the foregoing description, further, it will be seen that broadlytuned circuits are provided in connection with a balanced detectorlimiter which operates in conjunction with an oscillator for heterodynelimiting and that a balanced output connection is provided with which isconnected a suitable demodulator for preventing interruption in signalreception by the presence of a strong undesired carrier wave.Furthermore, there may be provided tuned circuits following therectifier for transmitting a new intermediate frequency and a third,rectifier or demodulator for providing the audio frequency signal.

From the foregoing description, it will be also seen that the method ofobtaining selectivity in a radio receiver for the purpose of eliminatinginterference may comprise the use of broadly tuned signal receivingcircuits preceding a limiter device and then demodulating the modulatingeffects of the limiter and also further using selective circuitsfollowing the demodulator to remove the local signal. If highlyselective circuits are used before the limiter, it has been found thatthe interference is not properly reduced,

I claim as my invention:

1. A selective radio receiving system comprising, in combination, abroadly tuned signal receiving circuit, a balanced detectorlimitercircuit connected therewith, said circuit including a balanced outputimpedance network, means for introducing oscillations into one leg. onlyof said output network, means for deriving an output signal from saidnetwork comprising a beat frequency signal resulting from mixing anincoming carrier wave and said oscillations in one leg of said network,and means for rectifying the beat frequency output signalderived fromsaid net- 2.'A selective radio receiving system in accordance with claim1, further characterized by the fact that the detector and therectifying means include diode rectifiers whereby a high degree ofstraight line rectification is provided.

3. In a radio signal receiving system, means for imparting selectivitythereto while eliminating undesired signalivoltages therefrom, whichcomprises a balanced limiter rectifier device, means for applyingoscillations thereto to provide a beat frequency signal output, meansfor rectifying said beat frequency output, a tuned selective circuit forreceiving thesignal output from said last named rectifier means, andmeans for detecting a desired signal component of said output waveconnected with said last named tuned output circuit. 1 r

4. A radiosignal receiving system in accordance with claim 3, furthercharacterized by the fact that the last named detector is an audiofrequency detector, and that an audio frequency amplifier and signaloutput device is'connected therewith for reproduction of a desiredsignal envelope, V

5. The vmethod of obtaining selectivity in a heterodyne beatlimiter'system, which comprises deriving a signal wave bybroadlyselecting the same, applying local oscillations of a predeterminedamplitude to the selected signal wave to limit the amplitude of thereceived signal wave and interference by means of a heterodyne beat, andsubsequently demodulating the interference modulating effects of thelimiting process.

6. The method of obtaining selectivity in a heterodyne beat limitersystem, which comprises deriving a signal wave by broadly selecting thesame, applying local oscillations of a predetermined amplitude to theselected signal wave to limit the amplitude of the received signal waveand interference by meansof a heterodyne beat, subsequently demodulatingthe interference modulating effects of the limiting process, andsubsequently further selecting a portion of the output from thedemodulating process in a narrow frequency band. V

'7. In a radio receiver for eliminating disturbing effects, thecombination of a broadly tunable receiver circuit, two opposed detectorcircuits, an oscillator arranged to superimpose a voltage on only one ofsaid opposed detector circuits, means for adjusting the detectorcircuits, for producing rectified energy having the same Wave form andthe same phase relation between current and voltage, an output circuitconnected with equipotential points on said detector circuits, and'arectifier device and a second signal rectifier circuit connected withsaid output circuit.

8. A selective radio receiving system for eliminating disturbingeffects, comprising, in combination, signal'receiving means including areceiving impedance elements connected in balanced relation to eachother with said anodes, means for producing rectified energy in saidimpedance elements having the same wave form and the same phase relationbetween current and voltage, an oscillator for superimposing anoscillator voltage lower than a received signal voltage in one branch ofsaid output circuit, and means for detecting resultant modulatedinterference and signal beat voltages to derive an original signaland'oscillator beat voltage, said last named means being coupledto saidb'alanced'output circuit at substantially cam-potential points thereon,V r

9. A selective radioreceiving system in accordance with claim 8, furthercharacterized by the fact that the last named detecting means isprovided with an output circuit including selective coupling meansresponsive tosignals in a predetermined narrow band of frequencies.

l0. A selective radio receiving system in accordance with claim 8,further characterized by the fact that a desired super-audible signal isproduced in connection with the second detecting means having soundmodulation thereon and that additional means are provided fordemodulating said signal.

11. A selective radio receiving system for eliminating disturbingeffects, comprising, in combination, a balanced detector limitercircuit, limited means for applying an oscillator voltage thereto notgreater than an incoming signal voltage wherebythe output beat frequencybetween the two input voltages will have an amplitude dependent onlyupon the amplitude of the oscilnation, a balanced detector limitercircuit, lim-.

ited means for appplying an oscillator voltage thereto not greaterthanan incoming signal volt age whereby the output beat frequencybetween the two input voltages will have an amplitude dependent onlyupon theamplitude of the oscillator voltage, means providing a signalrectifier following said limiter circuit for detecting a resultantmodulated interference and signal beat frequency to derive the desiredsignal therefrom, anda selective signal coupling means connected incircuit following the last named signal rectifier means.

13. A selective radio receiving system for eliminating disturbingeffects, comprising, in combination, a balanced detector limitercircuit, means for applying an oscillator voltage thereto not greaterthan an incoming signal voltage whereby the output beat frequencybetween the two input voltages will have an amplitude dependent onlyupon the amplitude of the oscillator voltage, and means providing asignal rectifier following said limiter circuit for detecting aresultant modulated interference and signal beat frequency to derive ahigh frequency modulated signaltherefrom, a selective tuned circuitfollowing said rectifier, and a demodulator connected with said lastnamed circuit.

14. A selective radio receiving system including, in combination, aheterodyne signal limiter including a source of oscillations forlimiting an incoming signal, means broadly responsive to an incomingsignal for applying said signal to said limiter, said limiter includingdiode rectifier means providing two output anodes, a signal detectorimpedance-coupled to said rectifier, means whereby the coupling isresponsive to a relatively wide band of signal frequencies, and saiddetector including a second diode rectifier means including two anodes,means providing a balanced output circuit with said last named anodes,and means providing equi-potential connections with said output circuit.

15. In a radio receiving system, means for eliminating disturbingeffects, comprising a limiter circuit including diode rectifier meanshaving two anodes, a signal rectifier coupled to said limiter circuit,means for introducing oscillations in circuit with one anode of saiddiode rectifier means relative to the other anodeyand said detectorbeing coupled to substantially equal potential points on said limitercircuit, and means for rectifying the signal output from said limitercircuit thereby to demodulate a signal wave resulting from a receivedinterference wave, and said output circuit being broadly tuned to thebeat frequency between said interference wave and a desired carrier wavein addition to the beat frequency between local oscillations and aninterference wave.

16. In a selective radio'receiving system for eliminating disturbingeffects, the combination of a high frequency amplifier broadlyresponsive to a predetermined frequency band, a diode detector meanshaving a pair of output anodes, said detector being connected with saidamplifier to receive the signal output therefrom, means providing abalanced output circuit for said detector in connection with saidanodes, means for introducing oscillations in circuit with one anoderelative to the other anode to limit incoming signals, a second outputcircuit connected with substantially equal potential points on saidbalanced diode output circuit, means for detectingthe resultantmodulated interference and signal beat voltages connected with said lastnamed output circuit, and coupling means between said last nameddetector means and the limiter circuit substantially uniformlyresponsive within a predetermined wide frequency range, which includesthe said interference and signal beat voltages and'a signal andoscillator beat voltage.

LELAND E. THOMPSON.

